1. Field of the Invention
The invention relates generally to the measurement of the ratio between two electrical signals. More particularly, it relates to an indicator circuit for measuring a parameter sensed by an electrical transducer, having an electrical output signal whose ratio to an electrical input signal to the transducer is proportional to the parameter being sensed.
2. Prior Art
Variable reluctance pressure transducers, such as described in U.S. Pat. No. 3,308,411, to J. L. Roshala, issued Mar. 7, 1967, are used extensively in aircraft for sensing differential pressures of various fluids such as engine and transmission oil, fuel, bleed air and freon. Typically, these transducers include two, identical, adjacent, series-connected induction coils disposed along a common axis, which are connected in parallel with a center-tapped stationary coil in a moving coil, dynamometer type indicator across an alternating voltage supply to form an AC bridge. The moving coil of the indicator is connected between the common terminal of the series connected induction coils and the center tap of the indicator stationary coil to serve as the detector or galvanometer leg of the bridge circuit. The differential pressure to be measured is sensed by a flexible diaphragm having opposite sides acted upon by the two pressures. A magnetic armature carried by this diaphragm is slidably disposed within the series connected induction coils for axial movement therein. The relative values in the two series connected transducer induction coils are determined by the axial position of the magnetic armature. Thus, any change in the differential pressure being sensed by the diaphragm causes a corresponding change in the position of the diaphragm and the magnetic armature attached to it. This action changes the relative inductance values of the series connected induction coils, which unbalances the bridge circuit and changes the current flow in the moving coil of the indicator. This moving coil then rebalances the circuit by moving to new angular position, thereby indicating the differential pressure acting on the diaphragm of the transducer. The indicator also includes at least one hair spring which serves to return the pointer to zero in the event of a power failure.
Since this indicator is a null type instrument, any torque exerted by hair springs to return the pointer to its zero position will cause the indicator to be biased down scale. Consequently, these hair springs must be designed to produce a very small restoring force on the indicator pointer, so that the down scale biasing effect of these springs is small and does not substantially effect the indicator accuracy. The use of these delicate hair springs limits the magnitude of vibration that this indicator can withstand. For example, this type of indicator is not rugged enough to perform satisfactorily in the high vibration environment of helicopters.
Also, since a change in the impedance of any circuit element in this measuring bridge circuit will affect the current flowing in the other elements of the bridge, only one of these null-type indicators can be used with each pressure transducer. When two of these indicators are connected in parallel to operate with the same pressure transducer, there is an interaction between them such that these two indicators do not necessarily indicate the same pressure nor are the readings repeatable. For example, physically moving the pointer on one indicator can cause a change in indication on the other indicator.